1. Field of the Invention
The present invention relates generally to methods of cementing high temperature wells and cement compositions therefor, and more particularly, to such methods and compositions which are suitable for cementing high temperature wells containing carbon dioxide.
2. Description of the Prior Art
In the completion of high temperature subterranean wells and particularly in such wells containing carbon dioxide, e.g., geothermal wells, the use of conventional hydraulic cement compositions often results in early well failure. Because of the high static well bore temperatures involved often coupled with the presence of brines containing carbon dioxide, conventional hydraulic well cements rapidly deteriorate due to alkali carbonation, especially sodium carbonate induced carbonation. In geothermal wells which typically involve very high temperatures, pressures and carbon dioxide concentrations, conventional well cement failures have occurred in less than five years causing the collapse of the well casing.
It has heretofore been discovered that a cement material known as calcium phosphate cement formed by an acid-base reaction between calcium aluminate and a phosphate-containing solution has high strength, low permeability and excellent carbon dioxide resistance when cured in hydrothermal environments. It has also heretofore been discovered that a cement composition formed of calcium aluminate, fly ash, sodium polyphosphate and sufficient water to form a pumpable slurry has excellent strength and carbon dioxide resistance when cured in hydrothermal environments. While this cement composition has achieved success in cementing high temperature wells including those containing carbon dioxide, the cement material has been subject to high fluid losses when placed in the wells.
Thus, there are needs for well cement compositions useful in cementing high temperature wells including those containing carbon dioxide which have low fluid losses.